In a spread spectrum system, a transmitted signal is spread over a frequency band that is much wider than the bandwidth of the information being transmitted. Two techniques commonly used in spread spectrum systems are frequency hopping and direct sequence (DS) modulation. Frequency hopping involves shifting the carrier frequency in discrete increments, in a pattern dictated by a pseudorandom code. In direct sequence modulation, each bit of an information-bearing signal is modulated by a higher frequency, pseudorandom code signal. The modulation may simply comprise reproducing the input code signal when the information bit is one, and inverting the code signal when the information bit is zero. Each bit of the code signal, or each bit of the product signal obtained by modulating the information-bearing signal with the code signal is referred to as a "chip."
In a system using direct sequence modulation, the chip rate, i.e., the frequency of the pseudorandom code signal, is typically much higher than the bit rate of the information-bearing signal. The bandwidth occupied by the transmitted signal is directly determined by the chip rate. A receiver in a direct sequence modulated communication system includes means for producing the same pseudorandom code signal as that used by the transmitter, in the same time epoch. The code signal is employed to decode the transmitted data and extract the information-bearing signal, even in the presence of noise or jamming.
Applications of spread spectrum systems are various, depending upon characteristics of the codes being employed for band spreading and other factors. In direct sequence systems, for example, where the code is a pseudorandom sequence, the transmitted signal acquires the characteristics of noise, making the transmission indiscernible to any eavesdropper who is incapable of decoding the transmission.
In certain kinds of spread spectrum communication systems, the data to be transmitted through the system is provided at two or more different data rates. In such a system, it is necessary to either vary the modulation rate of the transmitter while maintaining the spreading ratio, or transmit null data to act as filler at the lower data rates. Both options have certain disadvantages. The variations in the modulation of the transmitted signal can be more readily detected by an eavesdropper, thereby decreasing the security of the communication system. On the other hand, transmitting null data at low data rates increases the amount of energy required for the transmission. Therefore, it would be advantageous to provide a technique for use in a spread spectrum system that does not require this trade-off.